On October 9, 1936, the Hoover Dam in Black Canyon on the Colorado River — the highest concrete arch dam in the United States — became operational. And today it continues to serve as a vital source of power for the American Southwest. Each year, about 4 billion kilowatt-hours of hydroelectric powerare shuttled from the dam to Nevada, Arizona, and California, providing power for some 1.3 million people.

The dam impounds Lake Mead, which is one of the largest man-made lakes in the world, and consists of an enormous wall of concrete — some 4.4 million cubic yards of the stuff — that reaches 726 feet up from the canyon floor and stretches 1,244 feet along its top edge. Over the top of the dam lies a two-lane road that offers drivers-by an opportunity to catch a close glimpse of the dam’s size and power.

Building the massive structure was no easy feat, even though the project was completed early and under budget. Indeed, its construction claimed the lives of nearly 100 people, and exhaust from vehicles used in the diversion tunnels is believed to have caused at least another 40 deaths from pneumonia.

At the top of the dam, water diverted from Lake Mead is channeled into pipes that drop 500 feet down to the hydroelectric power plant at the dam’s bottom. The plant houses 17 hydraulic turbines, which are turned by the falling water and, in turn, rotate a series of electric generators.

The drawing shows how the completed Hoover Dam works. The Nevada wall of the Black Canyon (to the left) is shown solid, but the Arizona wall (to the right) shows with broken lines what the internal structures behind the wall look like. Credit: Encyclopædia Britannica, Inc.

Hoover Dam has a 2,080 megawatt power-producing capacity and a rated capacity of 2,998,000 horsepower. Its reputation as the world’s largest hydroelectric power station, however, was surpassed just years after its opening in 1936. In fact, the largest hydroelectric dam in the United States is the Grand Coulee Dam in Washington, which opened in 1942 and has a nearly 6,500 megawatt power capacity. And dozens of other massive hydroelectric stations have opened since, including what is currently the world’s largest, the Three Gorges Dam on the Yangtze River in China, which has a total power capacity of 22,500 megawatts.